An optical fiber is obtained by so-called “drawing”. During draw, the bottom portion of an optical fiber preform is softened by heating, and the tension is applied to the softened portion of the preform to reduce its diameter. The preform is typically made from material such as quartz glass. In general, a drawing system for such a process is equipped with a drawing furnace to heat the optical fiber preform; a cooling device to cool the drawn glass optical fiber; a coating device to coat a resin around the drawn glass optical fiber; and a spooling device to spool the coated optical fiber. In addition, the drawing furnace is placed at the highest position possible from the ground to increase the travel distance for cooling the optical fiber and to increase the drawing speed for improving productivity.
However, when an optical fiber was drawn from a standard drawing system, sometimes cross section(s) of the optical fiber had non-isotropic circle (such as ellipse). Deviation of an optical fiber cross section from an isotropic circle is called “non-circularity”. Non-circularity of an optical fiber is defined as (difference in the maximum and the minimum diameters of an optical fiber cross section)/(average of the maximum and the minimum diameters). An optical fiber is non-circular if its non-circularity is other than zero.
The cause of such non-circular optical fiber is unevenness in softness of an optical fiber preform in the circumferential direction due to unevenness of temperature distribution in the heated optical fiber preform, which happens because it is difficult to perfectly match the center axis of a drawing furnace to that of the optical fiber preform. Also, if the heat value of a heating element of the drawing furnace is uneven in the circumferential direction, the optical fiber could have a non-circular cross section. When an optical fiber has a large non-circularity, it causes problems such as an increase in polarization mode dispersion (PMD) and an increase in connection loss due to misalignment of the cores when the fiber is connected to another optical fiber.
To prevent non-circular optical fibers, drawing furnaces include equalization mean to uniformly distribute temperature along the heater circumference direction has been disclosed in U.S. Pat. No. 6,546,760. On the other hand, an optical fiber drawing method, which sets a relationship L (mm)≧5D (mm)−50 (mm), has been disclosed in Japanese Patent Application Laid-open No. 2004-224587. Where D is a diameter of an optical fiber preform and L is the length of the heat zone developed by the heater(s) along the drawing direction.
However, even with the methods disclosed in the U.S. Pat. No. 6,546,760, it is difficult to uniformly distribute the temperature around the circumference of the heater, and the structure of electrode within a furnace becomes complex. And, when the structure of the electrode is complex, it causes unevenness in the circumferential direction of the heating element, which causes uneven distribution of the heat. Further, with the methods disclosed in the Japanese Patent Application Laid-open No. 2004-224587, as the heater(s) is extended, the furnace itself needs to be extended, and it becomes difficult to receive an optical fiber preform into the drawing furnace, or the height of the drawing furnace and the distance for cooling down the drawn fiber are reduced. Those above problems are especially visible in a large optical fiber preform, such as a preform with a diameter of 100 mm or more.